Kit Harington is a direct descendant of Robert Catesby on his mother's side. Harington's full birth name is Christopher Catesby Harington.
In real life, Lord Robert Cecil was hunchbacked, and modern day physicians have diagnosed him having had scoliosis. This explains his irregular posture in the show. Cecil was 5'4" tall, but is played by Mark Gatiss, who is 6'1" tall.
Sir Willian Stanley (1548-1630) was denounced by Fawkes but pardoned by Sir William Cecil. He was a descendant of Sir William Stanley, whose late intervention in the Battle of Bosworth Field on August 22, 1485 won the crown for Henry Tudor (who became King Henry VII).
Part of the series was filmed at Oakwell Hall, located in Birstall, West Yorkshire.
Kit Harington, Mark Gatiss and Kevin Eldon appeared in Game of Thrones (2011) as Jon Snow, Tycho Nestoris and Camello/Goldcloak respectively. Although, they never share any scenes together in the series.